To change the default WordPress logo on the login page, make your logo fit in a 365px x 65px transparent png and then upload the image preferably into your theme images folder. In your theme functions.php file, add the following code below.

////////// Begin change site login logo
function my_login_logo() { ?>
	<style type="text/css">
    	#login h1 a, .login h1 a {
        	background-image: url(<?php echo get_stylesheet_directory_uri(); ?>/images/site-login-logo.png);
			height:65px;
			width:320px;
			background-size: 320px 65px;
			background-repeat: no-repeat;
			padding-bottom: 30px;
		}
	</style>
<?php }
add_action( 'login_enqueue_scripts', 'my_login_logo' );
 
// Change site login logo link
function my_login_logo_url() {
    return home_url();
}
add_filter( 'login_headerurl', 'my_login_logo_url' );
 
function my_login_logo_url_title() {
    return 'Your Site Title';
}
add_filter( 'login_headertitle', 'my_login_logo_url_title' );
////////// End change site login logo